Massacre's activation can be faintly heard anywhere on the map, with the voices speaking while active being Aatrox's voice-over lines played in reverse.
Enemies hit by Massacre will leave a blood trail between themselves and Aatrox that he will rapidly absorb (this is a remaining visual effect from PBE testing, where Aatrox would gain attack speed based on how many enemies had been hit by Massacre, that was recycled in V5.6)
Aatrox is a word play on Latin atrox "fierce, savage, cruel" in turn from ater "dull black, dark", from root PIE*h₂eh₁ter- "fire"; all referencing the Darkin weapon's physical manifestation and demonic nature (punning on dark kin), various historical fire-based weapons, the scorched earth strategy, and his 'war as an artform' philosophy. Latin atrox gives rise to Anglo-French atrocity.
In-universe, Aatrox is likely the closest humans' pronunciation of the Darkin's real-name, whose meaning remains unknown.
Aatrox is actually the Darkin once imprisoned in the sword. Aatrox' current body belongs to the thoroughly corrupt human wielder.
Tryndamere is suspected to have been chosen as Aatrox's new host, the attack on his village to serve as proof of his worth ("Tryndamere, my greatest creation") and Demonblade Tryndamere [S|L] as the potential end result of merging unhinged bloodlust and unbridled rage into a single vessel ("This one's rage will shape the world")
↑DeVaan, M. Etymoligical Dictionary of Latin and other Italic languages, p. 60